Table Of Content,
/. Raptor Res. 27(3):167-169
© 1993 The Raptor Research Foundation, Inc.
Diet of the Red-backed Buzzard
(Buteo polyosoma exsul) and the Short-eared Owl
{AsiO FLAMMEUS SUINDa) IN THE JUAN FERNANDEZ
Archipelago off Chile
Marcelo a. Fuentes,' Javier A. Simonetti,’’^
M. SOLEDAD SepUlVEDA^ AND PATRICIA A. ACEVEDO*
Departamento de Ciencias Ecologicas, Facultad de Ciencias, Universidad de Chile,
^
Casilla 653, Santiago, Chile
^Department ofInfectious Diseases, University ofFlorida,
Gainesville, Florida 32611 U.S.A.
The Juan Fernandez Islands (Robinson Crusoe, Santa eared Owls (seven from Santa Clara Island and 13 from
Clara and Alejandro Selkirk) off the Chilean coast hold a Robinson Crusoe Island, as well as prey remains found
unique biota featuring high levels of endemism in both scattered around a nest). Prey remains were identified to
plants and terrestrial animals (Castilla 1987, Stuessy et the finest possible level of resolution. Identification of
al. 1991). Four raptors inhabit the islands: Peregrine Fal- mammalian species was attained by comparison with a
con (Falco peregrinus) American Kestrel or Cernicalo de reference collection in the Seccion Mamiferos, Museo Na-
Juan Fernandez (F. sparverius), Red-backed Buzzard or cional de Historia Natural, Santiago.
Blindado {Buteopolyosoma), and Short-eared Owl or Nuco Thirty-six vertebrate prey items were identified from
{Asiaflammeus). Of these, two are endemic subspecies: F. pellets of Red-backed Buzzards, as well as seven inver-
sparverius fernandensis and B. polyosoma exsul (Johnson tebrates (three coleopterans, three arachnids, and onedeca-
1965, Schlatter 1987). These species are threatened by pod). Given the proportion of biomass contributed by ver-
illegal hunting and nest ravaging by introduced mammals tebrates, we focused our analysis on this prey category
(CONAF 1976). In general, Chilean raptors are decreas- (Table 1). The most frequent prey were rodents (47.3%),
ing due to illegal hunting, habitat alteration, and prey particularly the introduced house mouse {Mus musculus),
reduction (Jaksic and Jimenez 1986). Even the Short- whereas birds, largely petrels, accounted for 39.8% of the
eared Owl is categorized as inadequately known (Glade vertebrate prey. Interestingly, it has been suggested that
1987). In fact, the biology is poorly ifat all known (Schlat- attacks by Red-backed Buzzards may have forced petrels
ter 1987) for the raptors of Juan Fernandez Islands and to use burrows to reduce predation risks (Johnson 1965).
for all oceanic Chilean birds. While this behavior may decrease their vulnerability to
The diets ofJuan Fernandez’ Red-backed Buzzard and buzzards, it is ineffective toward introduced terrestrial
Short-eared Owl are cases in point. Although there are predators such as coatis {Nasua nasua) and cats {Feliscatus,
some data available on their respective diets in continental Torres and Aguayo 1971).
Chile (Schlatter et al. 1980, Rau et al. 1992), no such Red-backed Buzzards also preyed on European rabbits,
quantitative information exists on their diet in Juan Fer- which accounted for 5.6% of their vertebrate prey. Un-
nandez Islands. Anecdotal information suggests that Red- identified fishes made up 5.6% of the prey by number.
backed Buzzards prey on introduced rodents {Mus and Housse (1945) mentioned fish remains among the prey of
Rattus), poultry, young goats, and native petrels {Pterod- Buteo polyosoma polyosoma, suggesting that they were oc-
roma; Lonnberg 1921, Johnson 1965, Torres and Aguayo casional prey when the abundance of mammals and birds
1971). According to Saiz (1982), Red-backed Buzzards do was low. Fish may have been consumed as carrion. In
not consume European rabbits {Oryctolagus cuniculus). fact, Buteo polyosoma exsul was observed by one of us
Short-eared Owls, on the other hand, reportedly prey on (MSS) scavenging on dead fur seal {Arctocephalus philip-
Mus and Rattus and rabbits (CONAF 1976, Saiz 1982). pii) pups and eating placentae ofJuan Fernandez fur seals,
Herein, we describe quantitatively the diets of island Red- a suspected but so far unconfirmed behavior of this raptor
backed Buzzards and Short-eared Owls. (Torres 1987). Three pellets contained some hair attrib-
From November 1988 to April 1991 and in January utable to seals. Scavenging suggests that Red-backed Buz-
1992, we collected 26 pellets regurgitated by Red-backed zard is an opportunistic raptor.
Buzzards (22 from Alejandro Selkirk Island and four from In central Chile, B. polyosoma polyosoma preys largely
Robinson Crusoe Island), and 20 pellets cast by Short- on native rodents, secondarily on introduced rabbits, and
less frequently on native birds (Schlatter et al. 1980).
Therefore, this hawk’s diet in the Juan Fernandez Islands
is roughly similar to that in the continent, except for the
^ To whom all correspondence should be sent. consumption of fish and large mammal carrion.
167
168 Short Communications VoL. 27, No. 3
Table 1. Percent of prey in the diet of Red-backed Buz- changes in their resource spectrum (see Crespo 1966, Dal-
zard {Buteo polyosoma exsul) and Short-eared Owl {Asia by 1975). Therefore, if faced with a decline in the abun-
flammeus suinda) in the Juan Fernandez Archipelago. dance of exotic species by their control or eradication,
raptors may turn to native birds which already are threat-
ened. Alternatively, raptors may respond numerically, with
Prey Categories Buteo Asio
apopulation reduction coupled to the decline oftheir staple
Mammals
prey, which also represents a problem given the fragile
House mouse {Mus musculus) 41.7 0.0 conservation status of these raptors. This paradoxical
European rabbit {Oryctolagus situation should be considered when planning for the
cuniculus) 5.6 60.7 control of introduced rabbits and rats, a mandatory action
Rodentia: unidentified 5.6 10.7 if the survival of several unique species of the Juan Fer-
Mammal: unidentified 2.6 10.7 nandez Islands are to be saved from extinction.
—
Birds Resumen. En base al analisis de regurgitados, estudia-
Pterodroma 27.8 17.9 mos la dieta del Aguilucho de Mas Afuera (Buteo polyo-
Birds: unidentified 11.1 0.0 soma exsul) y del Nuco (Asia flammeus suinda) en el Ar-
chipielago de Juan Fernandez. El aguilucho consume
Fishes
principalmente muridos (introducidos) y aves. En menor
Fish: unidentified 5.6 0.0
medida depreda sobre conejos, consumiendo ademas peces
Total prey 36 28 y mamiferos marinos como carrona. El nucoconsume prin-
cipalmente conejos, fardelas y muridos. En ambas rapaces
destaca que la mitad de su dieta la componen especies
A total of28 prey items, all vertebrates, were determined exoticas.
for Short-eared Owl (Table 1). The most frequent were Acknowledgments
European rabbits, both adults and juveniles (eight and We are indebted to M. Ricci and Corporacibn Nacional
nine individuals, respectively). Secondarily, Short-eared Forestal (CONAF) personnel for their hospitality and
Owls preyed on birds, particularly both adults and eggs field assistance, and to F.M. Jaksic for comments to the
of petrels, as well as on unidentified rodents. In continental manuscript.
Chile, Short-eared Owls consume rodents almost exclu-
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Juan Fernandez Islands both Red-backed Buzzards and Myrceugenia fernandeziana y Sophora fernandeziana
Short-eared Owls preyed largely on introduced mammals: Tesis, Medicina Veterinaria, Universidad de Concep-
53% by number in the Red-backed Buzzard and 71% in cion, Chilian, Chile.
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1
J Raptor Res. 27(3);169-171
© 1993 The Raptor Research Foundation, Inc.
Observations of Wintering Gyrfalgons {Falco rusticolus)
Hunting Sage Grouse {Centrocercus urophasianus)
IN Wyoming and Montana U.S.A.
Christopher S. Garber
Wyoming Natural Diversity Database, The Nature Conservancy,
WY
3165 University Station, Laramie, 82071 U.S.A.
Brian D. Mutch
The Peregrine Fund, Inc., 5666 West Flying Hawk Lane, Boise, ID 83709 U.S.A.
Stephen Platt
Wyoming Department ofEnvironmental Quality,
WY
250 Lincoln St., Lander, 82520 U.S.A.
Little has been written on the prey preferences of win- preying upon this species. However, captive Gyrfalcons
tering Gyrfalcons {Falco rusticolus) outside of their breed- flownin falconry are reported to be ableto kill Sage Grouse
ing range (Palmer 1988, Dobler 1989, Sanchez 1993). (Christopher and Hardaswick 1988).
Most summaries ofGyrfalcon prey selection are from data Between 8 November and 6 December 1992, CSG and
collected during the breeding season (Sherrod 1978). Al- SP observed an individual Gyrfalcon on 10 separate days
though wintering Gyrfalcons occur within the range of (identification was based on comparisons of photographs)
Sage Grouse {Centrocercus urophasianus'. Cade 1982, 14 km east of Lander, Wyoming. The Gyrfalcon appeared
Johnsgard 1983), we found no reports of wild Gyrfalcons to be an adult female based on its plumage, yellow feet.
